We did - a card with their rsvp & any dietary requirements as well as an addressed envelope. No stamps though.
I do think it really helped to get people responding quicker - we invited 180 to our wedding & in the end had 160 guests.
I got an invite in this week for a wedding & have to wait for OH to go up town 2moro to buy an rsvp card which we'll post over wkend. If it had included one we would have rsvp'd the next day.
An Old Married 
"Women are meant to be loved, not to be understood." - The Sphinx Without a Secret by Oscar Wilde